1. Field of the Invention
This invention relates to an apparatus for evaluating semiconductor devices and more particularly to an apparatus which facilitates rapid, nondestructive identification of lattice defects, including strains generated in crystalline semiconductor wafers during the fabrication of semiconductor devices.
As can readily be understood by those familiar with the fabrication of semiconductor devices, each semiconductor wafer is subjected to numerous processing steps during fabrication which tend to introduce defects in the crystalline lattice thereof. These steps include subjecting the wafers to relatively high temperatures in the presence of controlled environments for purposes of enhancing oxide growth, impurity deposition, impurity diffusion, and/or annealing. Lattice defects thus introduced have a predictable effect, to a greater or lesser degree, on the probability that a properly functioning device will be produced, as well as on the overall reliability of that device.
It is therefore advantageous for a manufacturer of semiconductor devices to know which of the processing steps tend to produce lattice defects, and to what degree such defects influence performance so that each processing step which produces lattice defects can be modified to correct improper procedures.
2. Description of the Prior Art
Most presently known techniques employed in determining the degree, distribution, and type of lattice defects present in a given semiconductor wafer are destructive. Consequently, any study relying upon such techniques to determine the effects of lattice defects on the performance of semiconductor devices is necessarily highly statistical. Thus, in order to obtain useful data when employing destructive evaluation techniques, large numbers of wafers must be sacrificed for analysis purposes and considerable effort must be devoted to correlating the resulting data. Hence, it can be appreciated that nondestructive techniques are preferred over those techniques in which a destruction of specimens is required.
One technique heretofore employed in examining semiconductor wafers is a technique referred to as x-ray topography. When employing such a technique it is possible to determine the degree, distribution, and type of existing lattice defect, while no permanent change in the physical, chemical or electrical properties of the semiconductor wafer is introduced.
Studies using this technique, unfortunately, also are statistical in nature. However, the number of specimen samples required for a given confidence level is considerably reduced, since knowledge relating to lattice defects in a given semiconductor wafer may be obtained before and after each processing step.
Although numerous attempts have been made to identify lattice defects using x-ray diffraction, no totally satisfactory solution has resulted from these efforts. For example, the Berg-Barrett technique was first introduced about 1945 in which diffracted rays are caused to leave specimens on the same side as the incoming rays enter. In most current applications of this technique, the specimen and the film are supported in a stationary relationship during exposure. Another technique is the so-called Lang method which was first introduced in 1959 in which the diffracted rays travel through the specimen in reaching a photographic plate. This technique normally requires the specimen and photographic plate be translated to and fro during exposure in order to form an image of a relatively large portion of the specimen. In each of the aforementioned techniques, only an extremely small portion of the total radiation from a small x-ray source is used in the production of an x-ray topograph, thus the costs per x-ray topograph are undesirably high.
Of course, one of the major reasons that x-ray topography is not more widely used as an analytical tool for detection of lattice defects in semiconductor wafers is the high cost per x-ray topograph. In addition to the foregoing reasons for high costs per topograph, high cost of production of a useful x-ray topograph also is due to factors such as the relatively expensive equipment and materials required, as well as in the production thereof, and the necessity that highly trained personnel be employed over relatively long periods.
It is therefore the general purpose of the instant invention to provide an apparatus through a use of which reductions in costs per x-ray topograph of semiconductor wafers is realized.